Resources for a sustainable future

30/1/2006 BBC Scientists predict increasing droughts, floods and extreme weather and say there is growing evidence that human activities are to blame. The BBC News website looks at key questions behind climate change and global warming. What is climate change? The planet’s climate is constantly changing. The global average temperature is currently in the region of 15C. Geological and other evidence suggests that, in the past, this average may have been as high as 27C and as low as 7C. But scientists are concerned that the natural fluctuation has been overtaken by a rapid human-induced warming that has serious implications for the stability of the climate on which much life on the planet depends. What is the “greenhouse effect”? The greenhouse effect refers to the role played by gases which effectively trap energy from the Sun in the Earth’s atmosphere. Without them, the planet would be too cold to sustain life as we know it. The most important of these gases in the natural greenhouse effect is water vapour, but concentrations of that are changing little and it plays almost no role in modern human-induced greenhouse warming. Other greenhouse gases include carbon dioxide, methane and nitrous oxide, which are released by modern industry, agriculture and the burning of fossil fuels. Their concentration in the atmosphere is increasing – the concentration of carbon dioxide has risen by more than 30% since 1800. The majority of climate scientists accept the theory that an increase in these gases will cause a rise in the Earth’s temperature. What is the evidence of warming? Temperature records go back to the late 19th Century and show that the global average temperature increased by about 0.6C in the 20th Century. Sea levels have risen 10-20cm – thought to be caused mainly by the expansion of warming oceans. Most glaciers in temperate regions of the world and along the Antarctic Peninsula are in retreat; and records show Arctic sea-ice has thinned by 40% in recent decades in summer and autumn. There are anomalies however – parts of the Antarctic appear to be getting colder, and there are discrepancies between trends in surface temperatures and those in the troposphere (the lower portion of the atmosphere). How much will temperatures rise? If nothing is done to reduce emissions, current climate models predict a global temperature increase of 1.4-5.8°C by 2100. Even if we cut greenhouse gas emissions dramatically now, scientists say the effects would continue because parts of the climate system, particularly large bodies of water and ice, can take hundreds of years to respond to changes in temperature. It also takes greenhouse gases in the atmosphere decades to break down. It is possible that we have already irrevocably committed the Greenland ice sheet to melting, which would cause an estimated 7m rise in sea level. There are also indications that the west Antarctic ice sheet may have begun to melt, though scientists caution further research is necessary.